1. Field of the Invention
The present invention relates to a dry method of purifying an flue gas, and more particularly, it relates to a dry method of purifying an flue gas containing SO.sub.x, NO.sub.x, steam and oxygen.
2. Description of the Background Art
Flue gas discharged from a boiler or an incinerator typically contains toxic acid substances such as sulfur oxide (SO.sub.x), HCl, HF and nitrogen oxide (NO.sub.x) generally in concentrations of 10 to 3000 p.p.m. It is compulsory to remove such substances for environmental protection. SO.sub.x, HCl and HF are generally removed by a method of scrubbing the flue gas with an alkaline absorbing solution containing lime or the like. NO.sub.x is generally removed by an improved combustion method or a catalytic reduction method employing a catalyst. However, all of these methods are disadvantageous in that they increase the treatment cost.
The aforementioned wet process may be replaced by a simple semidry process, which can be carried out at a low cost. A method of improving the semidry process was reported in a 1986 symposium sponsored by EPA and EPRI. According to this method, the temperature of the flue gas, which is higher by 10.degree. to 20.degree. C. than that in the wet process, is 65.degree. to 75.degree. C. In this process, a scaling trouble is caused in the flue duct or if the flue gas is insufficiently diffused at the outlet of the stack. Further, unstable waste of Ca(OH).sub.2 --CaSO.sub.3 results from this method, to exert bad influence on the environment.
In relation to such a background, each of Japanese Patent Laying-Open Gazettes Nos. 209038/1986, 97640/1987, 183839/1987, 213842/1987 and 254824/1987, and Thermal and Nuclear Power Engineering Society, Hokkaido Branch Symposium, Mar. 19, 1986 discloses a new dry method of purifying a flue gas. According to such a new method of purifying a flue gas, a substance is prepared by adding water to a mixture of fly ash, lime and sulfuric acid or gypsum kneading the mixture, and subjected it to steam curing, thereby to prepare a compound sorbent. This compound sorbent is granulated to be 3 to 10 mm in diameter, for example, and filled in a moving bed. A flue gas of a temperature of 50.degree. to 230.degree. C., containing SO.sub.x, NO.sub.x and steam, is passed through this moving bed, to be subjected to simultaneous removals of SO.sub.x and NO.sub.x. According to this method, the SO.sub.x removal efficiency is at least 90%, while the NO.sub.x removal efficiency is 20 to 70%.
It has been reported that, according to the conventional dry method, the SO.sub.x removal efficiency was 93% when a moving bed was filled with a porous compound sorbent, which was granulated to be 3 to 10 mm in particle diameter, of 2 m.sup.3 (1.2 ton) and a flue gas containing SO.sub.x of 290 to 520 p.p.m. was passed through the moving bed at a flow rate of 1000 Nm.sup.3 /h. The stoichiometric ratio, i.e., the ratio Ca/SO.sub.x of an effective mole number of Ca contained in the compound sorbent to that of SO.sub.x contained in the flue gas, is 1.
According to this conventional method, however, the apparatus employed therefor must have a large capacity because of a low processing speed. Further, the apparatus is complicated since it is necessary to provide a pressure drop in the moving bed. It is reported that, as a result, the treatment cost is not much reduced as compared with the wet process but is about 75% of that in a conventional wet limestone method.